Diffuser or absorber behind listening position....and is it worth doing??

OK, first off thank you all for contributing your insight and suggestions. Especially those that were considerate enough to circle back and answer additional questions!

So, I'm definitely ordering some absorption panels for behind the listening position to cover a 4x6 ft space and 4 inches thick. Main objective achieved!

In addition, I'll be researching a light weight diffuser that's practical to implement covering the T.V. located between the speaker on the front wall when listening (any recommendations?)

Lastly, I'm also strongly considering setting up temporary free standing acoustic panels ( I have photo lighting stands I could repurpose) for managing the side wall's first reflection pionts when listening as an experiment. I just need to determine if all the set up is going to be more of a determent or a benefit to simply enjoying music.....which is the really the point and don't want to lose sight of that.

Thanks Again

I have a room almost exactly the same size (11 feet by 17.5 feet).  Diffusers can be beneficial.  However, be aware that QRD type diffusers (quadratic) need very careful implementation.  QRD diffusers can do weird things to the sound if they are placed very close to your listening position (like within 6 feet).  In this situation, the QRD diffusers can boost some midrange frequencies while cancelling out other midrange frequencies.  This creates a very bad and unbalanced sound. (this was from testing QRD diffusers on the side walls).

I was successful in placing two 2' x 2' QRD diffusers in the middle of the back wall.  That was the only place these QRD diffusers really worked.

I also have two 703FRK panels in the rear corners and another two 703FRK panels mounted at an angle on the front wall / ceiling edge.  These panels are actually "reflectors" for mids/highs due to the foil on the front surface. 

The rest of the panels are absorbers or tuned membrane bass traps.

The variety of "diffusers" that GIK applies onto their panels are not really true diffusers.  They are primarily "reflectors" to prevent much of the sound from being absorbed, but still keeping the panel useful for bass frequencie absorption.

The GIK "gridfuser" is sort of a QRD diffuser but without the channel slots.  Their PolyFuser is like a broadband reflector that reflects all angles (sort of works like a diffuser but more forgiving).

Be careful with how much absorption you add.  You can definitely over damp a room and suck the life out of the high frequencies (thereby removing excitement).

As an option to consider, Jeff Hedback is a designer who has created several hundred studios and also a large number of high end personal listening space designs. I've used several of his studio designs myself. Jeff basically works from his home, employs remote techniques to read your room acoustics, and works with you to create a custom design. The design comes to you in the form of architectural drawings and specifics as to materials and techniques. He then monitors the progress and results with more remote measuring methods. Award winning work and a nice guy to talk with. http://www.hdacoustics.net/

https://ua-acoustics.com Way less $ than GIK but not as pretty.

Or buy a 12 pack of  2' x 4' x 2" Owens Corning #703 rigid panels and experiment. $130.

Try 2 front, 2 back and one each side per Eric's instruction.

I don't know if treatments will provide the sound staging improvement you are looking. It will clean up the
"Smearing" effect you get when the same sound is reaching
your ears to close together in time.

In my mind this comes well before any other considerations.

You are at a very great/exciting/rewarding stage of Audiophilia
as the improvements in front of you now will make make the
biggest SQ improvement you are likely to achieve.

Please let us know your results!

Blankets and pillows can be your friend when experimenting.  Cover your TV, and put up a make shift absorber on the sides. 

If you do put something on the sides, again, due to lack of materials, go for a thick panel, but now you should consider diffusion as well. Look at the GIK impression series.  They can come with feet, so you could for instance put a diffusor in front of your TV while listening and move it out of the way when you need to.